1. Field of the Invention
The present invention relates to quantum imaging using correlated or entangled beams of light to enhance image processing and to the use of squeezed light in interferometry.
2. Description of Prior Art
The field of “quantum imaging,” using correlated or entangled beams of light to enhance image processing, holds promise for noiseless amplification of images and improvements in both the obtainable resolution and weak-image detection sensitivity. Many of these applications have already been demonstrated in some form using chi(2) nonlinear crystals. These demonstrations have typically been substantially limited by the need to angle-tune the nonlinear crystal to phase match a small range of spatial frequencies, or by the placement of the non-linear crystal inside of a resonant cavity, which again restricts the usable spatial modes.
Quantum information processing can be carried out with continuous variables in a parallel fashion to the ways in which binary data is processed. The basis of this approach is to use physical quantities with a continuous spectrum such as the quadrature amplitudes of a light field, rather than binary quantities such as the polarization state of the field.
Continuous variable quantum systems can involve many photons in one light mode, and this potentially has some advantage over single-photon systems. The possibility of higher data rates and simpler processing based upon standard telecommunication techniques exists. A continuous variable quantum communication system can be based on homodyne detection instead of single-photon detection. This has the advantage of producing a measurement outcome for each pulse of light rather than the probabilistic measurements of single-photon detectors.
An essential tool for implementing quantum information processing is some sort of memory for the storing the quantum bits, or qubits. A storage element for light, and in particular a quantum memory that retains the information regarding the detailed quantum state of the light is useful for quantum communication applications as well as quantum information processing protocols. The basic idea behind an implementation of a quantum memory is to slow the propagation of a pulse of light through a medium by controlling the linear dispersion of the medium at the frequencies near the central frequency of the light pulse in question. A narrow gain feature has accompanying it a dispersion or change in the index of refraction in the frequency neighborhood of the gain. The central region of the dispersion feature has a linear change in the index of refraction, which leads to a reduced group velocity for the pulse propagation. By changing the gain of the medium by controlling the pump intensity the slope of the linear dispersion region changes, thus changing the group velocity. Reducing the pump intensity to zero while the pulse of light is contained within the medium can cause the state of the light in the optical pulse to be stored in the ground-state coherences of the atomic populations that result from the ultimate absorption of the pulse as the pump light is turned off. The light pulse can then be “reconstituted” with all of its original quantum state information by turning the pump light back on. The mixing of the atoms by their thermal motion will cause the information to eventually become scrambled and lost to this decoherence mechanism. Similar optical memories have been demonstrated based on the concept of electromagnetically-induced transparency [see, for example, D. Phillips, A. Fleischhauer, A. Mair, R. Walsworth, and M. Lukin, “Storage of light in atomic vapor,” Phys. Rev. Lett. 86, 783 (2001)], wherein a transparency feature within an region of absorption in an atomic vapor is manipulated in a similar way.
A number of other information-processing protocols have been demonstrated, including quantum cloning and the demonstration of continuous-variable coherent-state quantum key distribution.
A discussion of quantum imaging applications is given in the following book: [“Quantum Imaging,” M. Kolobov, ed., (Springer, N.Y., 2007)]. A book on quantum information processing in this context is: [“Quantum Information with Continuous Variables of Atoms and Light,” N. J. Cerf, G. Leuchs, and E. S. Polzik, eds., (Imperial College Press, London, 2007)]. A review article on quantum information and communication applications is: [“From quantum cloning to quantum key distribution with continuous variables: a review,” N. Cerf and P. Grangier, J. Opt. Soc. Am. B 24, 324 (2007)].
A process to produce strongly squeezed light is desired. Particularly, a process to produce strongly squeezed light without the use of a cavity, in multiple spatial modes, and at low temporal frequencies, is desired.
While four-wave mixing was the first nonlinear optical process to be used to demonstrate the generation of squeezed states of light, the amount of squeezing obtained by this technique has been quite modest. Generally, only about −1 dB or less of squeezing has been obtained through four-wave mixing (4WM) processes, with the strongest effect of −2.2 dB obtained with the use of a laser-cooled atom source.
Optical parametric oscillators (OPOs) with a cavity built around a chi(2) material have produced much better and more reliable squeezing. Up to −9.7 dB of intensity-difference squeezing has been measured using an OPO. However, a disadvantage of OPOs for imaging applications is the requirement of a cavity. Also, up to −10 dB of quadrature squeezing has been observed from an OPO.
Parametric down-conversion (PDC) with a chi(2) material has the advantage of typically being used in a single pass geometry, but requires a high-intensity, pulsed pump source to obtain any significant gain.